Buggy code:
public class StrBuilder implements Cloneable { static final int CAPACITY = 32 ; private static final long serialVersionUID = 7628716375283629643L ; protected char [ ] buffer ; protected int size ; private String newLine ; private String nullText ; public StrBuilder ( ) { } public StrBuilder ( int initialCapacity ) { } public StrBuilder ( String str ) { } public String getNewLineText ( ) { } public StrBuilder setNewLineText ( String newLine ) { } public String getNullText ( ) { } public StrBuilder setNullText ( String nullText ) { } public int length ( ) { } public StrBuilder setLength ( int length ) { } public int capacity ( ) { } public StrBuilder ensureCapacity ( int capacity ) { } public StrBuilder minimizeCapacity ( ) { } public int size ( ) { } public boolean isEmpty ( ) { } public StrBuilder clear ( ) { } public char charAt ( int index ) { } public StrBuilder setCharAt ( int index , char ch ) { } public StrBuilder deleteCharAt ( int index ) { } public char [ ] toCharArray ( ) { } public char [ ] toCharArray ( int startIndex , int endIndex ) { } public char [ ] getChars ( char [ ] destination ) { } public void getChars ( int startIndex , int endIndex , char [ ] destination , int destinationIndex ) { } public StrBuilder appendNewLine ( ) { } public StrBuilder appendNull ( ) { } public StrBuilder append ( Object obj ) { } public StrBuilder append ( String str ) { } public StrBuilder append ( String str , int startIndex , int length ) { } public StrBuilder append ( StringBuffer str ) { } public StrBuilder append ( StringBuffer str , int startIndex , int length ) { } public StrBuilder append ( StrBuilder str ) { } public StrBuilder append ( StrBuilder str , int startIndex , int length ) { } public StrBuilder append ( char [ ] chars ) { } public StrBuilder append ( char [ ] chars , int startIndex , int length ) { } public StrBuilder append ( boolean value ) { } public StrBuilder append ( char ch ) { } public StrBuilder append ( int value ) { } public StrBuilder append ( long value ) { } public StrBuilder append ( float value ) { } public StrBuilder append ( double value ) { } public StrBuilder appendWithSeparators ( Object [ ] array , String separator ) { } public StrBuilder appendWithSeparators ( Collection coll , String separator ) { } public StrBuilder appendWithSeparators ( Iterator it , String separator ) { } public StrBuilder appendPadding ( int length , char padChar ) { } public StrBuilder appendFixedWidthPadLeft ( Object obj , int width , char padChar ) { } public StrBuilder appendFixedWidthPadLeft ( int value , int width , char padChar ) { } public StrBuilder appendFixedWidthPadRight ( Object obj , int width , char padChar ) { if ( width > 0 ) { ensureCapacity ( ( ( size ) + width ) ) ; String str = ( obj == null ) ? getNullText ( ) : obj . toString ( ) ; int strLen = str . length ( ) ; if ( strLen >= width ) { <START_BUG> str . getChars ( 0 , strLen , buffer , size ) ; <END_BUG> } else { int padLen = width - strLen ; str . getChars ( 0 , strLen , buffer , size ) ; for ( int i = 0 ; i < padLen ; i ++ ) { buffer [ ( ( ( size ) + strLen ) + i ) ] = padChar ; } } size += width ; } return this ; } public StrBuilder appendFixedWidthPadRight ( int value , int width , char padChar ) { } public StrBuilder insert ( int index , Object obj ) { } public StrBuilder insert ( int index , String str ) { } public StrBuilder insert ( int index , char [ ] chars ) { } public StrBuilder insert ( int index , char [ ] chars , int offset , int length ) { } public StrBuilder insert ( int index , boolean value ) { } public StrBuilder insert ( int index , char value ) { } public StrBuilder insert ( int index , int value ) { } public StrBuilder insert ( int index , long value ) { } public StrBuilder insert ( int index , float value ) { } public StrBuilder insert ( int index , double value ) { } private void deleteImpl ( int startIndex , int endIndex , int len ) { } public StrBuilder delete ( int startIndex , int endIndex ) { } public StrBuilder deleteAll ( char ch ) { } public StrBuilder deleteFirst ( char ch ) { } public StrBuilder deleteAll ( String str ) { } public StrBuilder deleteFirst ( String str ) { } public StrBuilder deleteAll ( StrMatcher matcher ) { } public StrBuilder deleteFirst ( StrMatcher matcher ) { } private void replaceImpl ( int startIndex , int endIndex , int removeLen , String insertStr , int insertLen ) { } public StrBuilder replace ( int startIndex , int endIndex , String replaceStr ) { } public StrBuilder replaceAll ( char search , char replace ) { } public StrBuilder replaceFirst ( char search , char replace ) { } public StrBuilder replaceAll ( String searchStr , String replaceStr ) { } public StrBuilder replaceFirst ( String searchStr , String replaceStr ) { } public StrBuilder replaceAll ( StrMatcher matcher , String replaceStr ) { } public StrBuilder replaceFirst ( StrMatcher matcher , String replaceStr ) { } public StrBuilder replace ( StrMatcher matcher , String replaceStr , int startIndex , int endIndex , int replaceCount ) { }

Fix:
str . getChars ( 0 , width , buffer , size ) ;